First Law of Thermodynamics question

in10sivkid

a quantity of gas in a cylinder recieves 1600J of heat from a hot plate. At the same time 800J of work are done on the gas by outside forces pressing down on a piston. Calculate the change in thermal energy of the gas

Input heat Q = 1600J
The amount of work done on the gas is, W = 800J

now i'm not exactly sure how I'm supposed to set this up

do I use U = Q - W
or Q = W + U

so i either get 800J or 2400J for the answer for U

i know its a simple question but i think i'm just a bit confused conceptually. any help would be great :)

Related Introductory Physics Homework News on Phys.org

Nylex

It should be $\Delta U = Q + W$, where Q is the heat input to the system and W is the work done on the system. In $\Delta U = Q - W$, W is the work done by the system.

geosonel

in10sivkid said:
a quantity of gas in a cylinder recieves 1600J of heat from a hot plate. At the same time 800J of work are done on the gas by outside forces pressing down on a piston. Calculate the change in thermal energy of the gas

Input heat Q = 1600J
The amount of work done on the gas is, W = 800J

now i'm not exactly sure how I'm supposed to set this up

do I use U = Q - W
or Q = W + U

so i either get 800J or 2400J for the answer for U

i know its a simple question but i think i'm just a bit confused conceptually. any help would be great :)
you are correct.
it's the first form:
ΔU = Q - W,
where ΔU is the change in internal energy, Q is the heat added to the system, and W is the work done by the system. of course, when work is done to the system (like your problem), W is negative, so that -W is positive, resulting in an increase in internal energy (like your problem):
ΔU = (1600) - (-800) = 2400 J
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/firlaw.html

Last edited:

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving